bashParameterExpansion.sh

bashParameterExpansion.sh

#!/bin/bash
# See
# https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html#Shell-Parameter-Expansion

# ${parameter:-word} substitutes word if parameter is unset
echo '${parameter:-word}'
dog="dog"
echo "${dog:-"cow"}"    # Prints "dog"
echo "${cow:-"cow"}"    # Prints "cow"
echo "${cow}"           # Prints nothing (cow is unset)


# ${parameter:=word} sets parameter equal to word if parameter is unset, then substitutes parameter
echo ''
echo '${parameter:=word}'
echo "${dog:="cow"}"    # Prints "dog"
echo "${cow:="cow"}"    # Prints "cow"
echo "${cow}"           # Prints "cow" 


# ${parameter:?word} Prints word on stderror if parameter is unset
echo ''
#echo '${parameter:?word}'
#echo "${cat:?"A cat related error"}"    # Prints "./bashParameterSubstitution.sh: line 17: cat: A cat related error" and returns 1


# ${parameter:+word} Prints word on stderror if parameter is unset
echo ''
echo '${parameter:+word}'
echo "${cat:+"a cat related success"}"  # Prints nothing since cat is unset
cat="garfield"
echo "${cat:+"a cat related success"}"  # Prints "a cat related success"

# ${parameter:offset:length}
string="0123456789abcdef"   # 16 values
echo "${string:1}"          # Cut off the first value
echo "${string:0:-1}"       # Cut off the last value
echo ${string:7:2}          # Offset by 7, length 2
echo ${string:7:-2}         # start offset by 7, end 2 from the end

# Prefix
echo ' '
metacow="metacow"
metacat="metacat"
metadoe="metadoe"
postdog="postdog"
postowl="postowl"
echo "${!meta*}"            # Prints "metacow metacat metadow"
echo "${!meta@}"            # Prints "metacow metacat metadow" as separate words
echo "${!post*}"            # Prints "postdog postowl"

# Length 
echo ' '
echo "${#metacow}"  # Prints "7"
echo "${#dog}"      # Prints "3"


# Delete from start
echo "${metacow#meta}"      # Prints cow

# Shortest matching pattern
echo "${postowl#*o}"        # Prints stowl 

# Longest matching pattern
echo "${postowl##*o}"       # Prints wl 

# A practical example - keep the file part of a path
mypath="/some/path/to/a/file"
echo "${mypath##*/}"        # Prints "file"


# Delete from the end
echo ' '
echo "${metacow%c[a-z][a-z]}"   # Prints meta

# Shortest matching pattern
echo "${metacat%a*}"            # Prints metac

# Longest matching pattern
echo "${metacat%%a*}"           # Prints met


# A practical example - keep the directory part of a path 
echo "${mypath%/*}"             # Prints /some/path/to/a